1. Field of the Invention
This invention relates to an improved process for making rare earth-barium-copper oxide superconductors with transition temperatures above 90 K.
2. Description of Related Art
Bednorz and Muller, Z. Phys. B64, 189-193 (1986), disclose a superconducting phase in the La-Ba-Cu-O system with a superconducting transition temperature of about 35 K. Samples were prepared by a coprecipitation method from aqueous solutions of Ba-, La- and Cu-nitrate in their appropriate ratios. An aqueous solution of oxalic acid was used as the precipitant.
Chu et al., Phys. Rev. Lett. 58, 405-407 (1987), report detection of an apparent superconducting transition with an onset temperature above 40 K under pressure in the La-Ba-Cu-O compound system synthesized directly from a solid-state reaction of La.sub.2 O.sub.3, CuO and BaCO.sub.3 followed by a decomposition of the mixture in a reduced atmosphere. Chu et al., Science 235, 567-569 (1987), disclose that a superconducting transition with an onset temperature of 52.5 K has been observed under hydrostatic pressure in compounds with nominal compositions given by (La.sub.0.9 Ba.sub.0.1).sub.2 CuO.sub.4-y, where y is undetermined. They state that the K.sub.2 NiF.sub.4 layer structure has been proposed to be responsible for the high-temperature superconductivity in the La-Ba-Cu-O system (LBCO). They further state that, however, the small diamagnetic signal, in contrast to the presence of up to 100% K.sub.2 NiF.sub.4 phase in their samples, raises a question about the exact location of superconductivity in LBCO.
Cava et al., Phys. Rev. Lett. 58, 408-410 (1987), disclose bulk superconductivity at 36 K in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 prepared from appropriate mixtures of high purity La(OH).sub.3, SrCO.sub.3 and CuO powders, heated for several days in air at 1000.degree. C. in quartz crucibles. Rao et al., Current Science 56, 47-49 (1987), discuss superconducting properties of compositions which include La.sub.1.8 Sr.sub.0.2 CuO.sub.4, La.sub.1.85 Ba.sub.0.15 CuO.sub.4, La.sub.1.8 Sr.sub.0.1 CuO.sub.4, (La.sub.1-x Pr.sub.x).sub.2-y Sr.sub.y CuO.sub.4, and (La.sub.1.75 Eu.sub.0.25)Sr.sub.0.2 CuO.sub.4. Bednorz et al., Europhys. Lett. 3, 379-384 (1987), report that susceptibility measurements support high-T.sub.c superconductivity in the Ba-La-Cu-O system. In general, in the La-Ba-Cu-O system, the superconducting phase has been identified as the composition La.sub.1-x (Ba,Sr,Ca).sub.x O.sub.4-y with the tetragonal K.sub.2 NiF.sub.4 -type structure and with x typically about 0.15 and y indicating oxygen vacancies.
Wu et al., Phys. Rev. Lett. 58, 908-910 (1987), disclose a superconducting phase in the Y-Ba-Cu-O system with a superconducting transition temperature between 80 and 93 K . The compounds investigated were prepared with nominal composition (Y.sub.1-x Ba.sub.x).sub.2 CuO.sub.4-y and x=0.4 by a solid-state reaction of appropriate amounts of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO in a manner similar to that described in Chu et al , Phys. Rev. Lett. 58, 405-407 (1987). Said reaction method comprises more specifically heating the oxides in a reduced oxygen atmosphere of 2.times.10.sup.-5 bars (2 Pa) at 900.degree. C. for 6 hours. The reacted mixture was pulverized and the heating step was repeated. The thoroughly reacted mixture was then pressed into 3/16 inch (0.5 cm) diameter cylinders for final sintering at 925.degree. C. for 24 hours in the same reduced oxygen atmosphere. The material prepared showed the existence of multiple phases.
Hor et al., Phys. Rev. Lett. 58, 911-912 (1987), disclose that pressure has only a slight effect on the superconducting transition temperature of the Y-Ba-Cu-O superconductors described by Wu et al., supra.
Arjomand et al., J. C. S. Dalton 1061-1066 (1975), disclose the preparation of BaCuO.sub.2.5 by heating an equimolar mixture of BaO.sub.2 and Cu(NO.sub.3) at 580.degree. C. for 24 hours.